Herein by “underwater” is meant below or under a surface of a body of water, whether moving or static, natural or man-made, e.g. a sea bed, ocean floor, river bed, canal bottom, lake or loch floor, dam floor, or the like. However, the invention finds particular use in seas or oceans.
“Mass flow” excavators operate by directing a flow of high volume fluid under low pressure at the sea bed or at a subsea structure or surface to displace material such as sea bed material. This is in contradistinction to “jet” type apparatus which direct a flow of low volume fluid under high pressure at the sea bed. “Mass flow” and “jet” or “jetting” are therefore distinct terms, known in the art. In terms of differences between mass flow excavators and jetting excavators, in mass flow (as the name suggests) it is the mass or volume of flow which moves or removes material. In jetting it is the speed, and thus pressure of the jets which does the cutting. In jetting pressures can be of the order of 3,000 psi (2.07×107 Pa), whereas mass flow excavators typically operate at pressures in the order of 10 to 20 psi (6.89×104 to 1.37×105 Pa).
It will appreciated that power is a function of pressure and flow rate. Therefore, for a given available power in order to transfer power from the device into seawater and into the soil to be disturbed, it is possible to select high flow rate and low pressure (i.e. mass flow) or to select high pressure and low flow rate (i.e. jetting).
A mass flow excavator is typically tethered from a vessel by means of a crane wire, which is used to lower and retrieve the excavator, and to maintain a given distance from the sea bed or structure or object requiring excavation, such as a subsea oil or gas pipeline. In order to control the excavator, sonar detection means can be used to allow the excavator operator to view the excavation in real time. Cameras and metal detection means can also be used to assist the operator.
Underwater mass flow excavation apparatus are known. For example, GB 2 297 777 A and WO 98/027286, also by the present Applicant (Assignee), the content of which is incorporated herein by reference.
Mass flow excavation is a means of creating cavities in the sea bed or deburying objects. In the trade of mass flow excavation it is accepted that excavated material is spread in a circular manner around the cavity. The material is displaced to a distance far enough to retain depth of the created cavity. There are, however, limits to the distance to which the material can be thrown, which then limits the size and depth of the cavity to be created. Current applications of mass flow excavation are restricted to those excavations which do not require the sea bed material to be excavated, collected and deposited in a particular area, such as is required for excavation of harbour areas or canals, where it is important that the excavated material is removed to particular locations.
The present Inventor has identified that where the excavation requires a large cavity to be created, in order to overcome this limitation in mass flow excavators a means is required to collect and carry the excavated material through a duct means away from the excavated cavity. The distance by or over which the material requires to be carried is determined by the size of the cavity to be created.
US 2007166107 (JACOBSEN et al) discloses a subsea excavation and suction device which includes a suction head with an inlet opening at an outer, free end and an outlet opening connected to a suction hose arranged at a distance from the inlet opening. The suction head is mounted on a hydraulic controller arm and has at the inlet opening provided with mechanical and hydraulic means to disintegrate solid material (sediment). The hydraulic means includes a number of jet nozzles, while the mechanical means includes bars. The cross-sectional area of the inlet opening is larger than the cross-sectional area of the outlet opening.
U.S. Pat. No. 4,479,741 A (BERTI et al) discloses a self-propelling device for burying and digging up subsea conduits laid on beds of an incoherent material. The device has: disintegrating members using high pressure water jets to create a slurry of material; digging members having suction members which draw the suspension prepared by the disintegrating members, thus leaving a trench behind; and displacement members for moving the device on the sea bed astride the conduit.
EP 1 857 598 A1 (IHC HOLLAND IE) discloses a suction dredger comprising a dredging tube which at one end carries a suction head and which at the other end is connected to the suction dredger hull through a hull pivot with a pivot axis which is generally transverse with respect to said hull.
www.toyopumpseurope.com/toyo exca.html discloses a submersible excavator having a mechanical agitator.
The above apparatus are mechanically complex and provide a slow means of excavation in comparison to their relative expense.
It is an object of at least one embodiment of at least one aspect of the present invention to seek to obviate or at least mitigate one or more of the aforementioned problems in the prior art.
It is an object of at least one embodiment of at least one aspect of the present invention to seek to obviate or at least mitigate one or more problems in the prior art.
It is an object of at least one embodiment of at least one aspect of the present invention to provide a means to effect a desire for excavating a location or “deburying” an object and optionally for collecting and transporting excavated material in a rapid and comparatively inexpensive manner.